An English auction house is making a mistake if it insists on selling the pistol used to kill outlaw Jesse James knowing that it is stolen property, the man from whose museum the gun was stolen in 1968 said yesterday.

The .44-caliber Smith & Wesson revolver, Serial No. 3766, was on loan from the late Henry G. Lingenfelder of Towson to Richard Hahn's Jesse James Museum in Sullivan, Mo., when the break-in occurred during a severe summer storm.

Edmund Greenwood, a spokesman for Wallis & Wallis, an auctioneer in Lewes, Sussex, said plans to sell the revolver would proceed despite yesterday's report in The Sun about the theft.

"If I were [the auctioneers], I wouldn't sell it," said Mr. Hahn, a St. Louis electronics manufacturer. "They're crazier than hell if they do. It's stolen property."

Mr. Hahn said he reported the theft to the Missouri State Police and "insisted" the complaint be filed with the FBI and Interpol.

"It should still be in their computers," he said.

The FBI in Washington, D.C., is searching for the 25-year-old report and notified Missouri police of the gun's reappearance, Baltimore FBI spokesman Andrew Manning said.

"I left him a message of my own that he'd better not sell the gun because it belongs to us," he said.

Mr. Greenwood, who identified the seller only as an American, said, "Our position is that we are satisfied with the right of the vendor to sell the weapon. We did, in fact, know that at one time it had been stolen, but it obviously has been recovered and is back on the market again."

Mr. Hahn and Mr. Lingenfelder derided Mr. Greenwoood's assertion that the gun had somehow acquired legitimacy over time and could be sold without challenge.

Mr. Lingenfelder's lawyer told him to send as much documentation as possible by overnight post to Mr. Butler, at Wallis & Wallis, and to Scotland Yard's Major Fraud Unit.

"I told him to tell them that they are dealing at risk because this is stolen property" belonging either to Mr. Lingenfelder or to the insurance company that paid the claim for it, said lawyer Robert Preller of Towson.

Mr. Hahn said he heard several years ago that "the gun had been bought under the counter and was in England."

Three years ago, Mr. Lingenfelder said, a man with a British accent called him, said the revolver was in England and asked if would he contest ownership.

When Mr. Lingenfelder said he would contest, the man hung up. He heard nothing of the gun until Monday, when The Sun reported its impending sale. "I find it incredible that they say they are going ahead with the sale after this," Mr. Lingenfelder said yesterday.

Mr. Lingenfelder's father bought the revolver in 1952 from the estate of E. Stanley Gary of Baltimore. Mr. Gary bought it in 1904 from Corydon F. Craig, son of the jailer in St. Joseph, Mo., where Bob and Charley Ford were held during their trial. Bob Ford shot Jesse James in the back of the head April 3, 1882, as his brother watched. The governor of Missouri pardoned the Fords, who had been sentenced to hang. Bob Ford gave the gun to his jailer for his kind treatment.